Carrier-wave transmission



July s, 1928. 1,675,893

J. S. JAMMER CARRIER WAVE TRANSMISSION Filed July 17, 1923 Miles Gain V //7l f7 for:

Jacob 5. Jam/77 er:

PatentedJuly 3, 1928.

UNITED STATES PATENT oFi-icE.

JACOB JAMMER, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COMPANY,

- INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CARRIER-WAVE TRANSMISSION.-

Application filed July 17, 1923. Serial K056520131.

This invention relates to carrier wave 7 transmission.

Amongthe objects of the invention are:

To provide a multi lex carrier wave system wherein a high egree of selectivity is obtained with a relatively small amount of apparatus.

To provide an efiicient repeater which is inexpensive to manufacture and maintains To provide a repeater whose gain is substantially independent of changing conditions of the line with which it is associated- To provide a two-way repeater whose gain is substantially independent of changes in theimpedan'ce characteristics of the transmission line sections coupled thereto.

To provide a two-way repeater which may be coupled to a transmission line without employing hybrid coils, balancing networks or directional filters.

To provide a repeater whose gain is independent of the frequency of the currents to beamplified thereby.

To provide a repeater whose gain is sub stantially constant over a wide range of frequencies.

In accordance with one feature of the invention, a multiplex carrier current telephone system, wherein diiferent carrier frequencies are employed for the various channels and the same carrier frequency is em ployed for the oppositely directed transmis-. sions over 'the" same channel, has at each terminal an artificial line or network individual to each channel for balancing the main line over a frequency range equivalent to ne side band only of the carrier.

In a'ccordance with another feature'of this invention, a carrier current telephone or other signaling system is provided with a two-way repeater having one or more amplifiers connected in a loop circuit which is coupled to the transmission line without em- .ploying hybrid coils, balancing networks or directional filters.

In the drawing: Fig. 1 shows diagrammatically a multi- Qplex carrier current telephone system pro- 'vided with a mid-line repeater having am plifiers whose input and output circuits areinductively coupled in a loop circuit.

to the respective carrier current channels. Where the geographical distance between terminal stations is suflicient to require it,

one or more mid-line repeaters R are connected to the transmission line ML.

The apparatus and circuits at'the two terminal stations are similar so thatapparatus and c1rcu1ts.for connecting only one low frequency line to a carrier current channel will be described, and whenever any distinctions appear they will be particularly pointed out.

The system disclosed has four channels, each operating with carrier current of a different fequency. The oppositely directed tansmissions over the same channel employ the same carrier frequency. Onl one side band of frequencies is transmitte ,the same side band of frequencies being transmitted in both directions.

The channel frequencies are separated at the terminal station by band filters BF BF BF and BF which may be of the t e disclosed in U. S. patent to Campbell 1%. 1,227,113, May 22, 1917. Each band filter is designed to pass onlycurrents of frequencies within the particular side band being transmitted over the channel with which the filter is associated, and to suppress currents of all other frequencies.

Low frequency line L,, extending to and terminating at a telephone switchboard (not shown), is coupled to the high frequency main linejML by means of transmitting and.

receiving channels TC and RC, connected to line L through a balanced transformer 1 and the main line ML through a balanced transformer 2.

The two channels T0 and RC are rendared su st n a y conjugate y the provi-v' sion of artificial lines or nets LN and MTQN to'balance the respective low frequency and high frequency lines. The net MLN merely balances'the high frequency line for the par-, ticular range of frequencies transmitted and received by the channels T0 and R0,.

The transmitting channel TC includes a modulator M which may be of any well known type, such, for example, as the electronic valve modulator disclosed in Fig. 42 of the. article on Carrier current telephony .and tele'graphy by Colpitts and Blackwell to prevent the circulation, in the loop circuit including the conjugately related transmitting and receiving channels TC and RC of currents of frequencies not balanced by the network MLN The demodulator DM may be of any well known t e, such, for example, as the electronic va ve demodulator disclosed in. Fig. 42 of the Colpi tts and Blackwell article, supra. Carrier currents are supplied to the demodulator DM from the source CS.

It will be noted that in the multiplex carrier current telephone'system of Fig. 1 only two band filters for each channel, one at each terminal station, are required. Since v each high He uency main line balancing network is-requ1red to balance for only the frequencies within the range of one side band, these networks are less expensive to manufacture and maintain. since only one sideband of frequencies is transmitted in both directions over a channel, greater frequency spacing of the-channels results for a given carrier spacing thereby affording greater selectivity. Consequently the band filters may be made less expensive as the frequencies against which each is required to discriminate are-spaced farther away from those which are to passed therethrough. I

The two-way repeater of Fig. 1 comprises an electronic amplifier A of the usual. three electrode type having an input trans'fornier' 3 and an output transformer 4. The second ary winding of the output transformer4 is connected through a resistance network '5 to the primary winding of the input transformer 3 by means of conductors 6 and Furthermore,

Thus, the am lifier is included in a loo circuit which is connected to the transmission line ML by means of conductors 8 and 9.

The resistance network 5 is adjusted to such a value that the loss. within the loop plus the amplified energy fed to' the transmission line by way of the connecting con: ductors 8 and 9 exceeds the total gain or amplification of amplifier A. The total gain or. amplification is the initial amplification plus the successive re-am lifications due to a portion of the ampli er output energy being fed back to the input of the amplifier. When the resistance network is so adjusted that the repeater can not sing, the amount by which the loss exceeds the gain may be termed the singing margin of the repeater. Thus singing margin may be defined as the difference between the total loop gain for a given condition and that loop gain which will just cause the repeater to sing. The various elements of the loop circuit should be adjusted so that the initial input energy to the amplifier and the amplified energy fed back from the output thereof are substantially in phase. M It has been demonstrated experimentally and proved mathematically that in a .repeater of the character shown in Fig. 1, the total ain thereof is substantially independent of the absolute gain of its amplifier and is a function of the singing margin. The

total gain to be obtained from the repeater A can therefore be-regulated by varying the values of the resistance network in tlieloop circuit. I

The total gain Q of the repeater may be expressed by the equation where M is an, index of the singing margin and K is a constant whose value determines the stability of the repeater gain. The constant K is a function of the ratio of the repeater input 'im edance to the impedance of the transmission line to which the repeater is connected.

The net gain of the repeater can be stabilized to a considerable extent throughout its working range by increasing the ratio of the repeater input impedance to the line impedance- The gain characteristic of the repeater is slfiolvcvn in Fi. 3 wherein for different values 0 of the repeater instandard cable miles as ordinates and singing margin in standard curves liavebeen plotted with the gain 4 l cablemiles asabscissae. I

These curves show that the gain of the rqpeater is very unstable for .v low values 0 K andis rather stable as a preaches the limiting value of 2- whenlsequals 2 the ratio of the' repeater input impedance to upon the gain to be derived from the repeater so that the repeater is very stable.

In order that the repeater may be rendered stable, it is desirable that the input impedance of the input transformer 3 be made high as compared to the impedance of whose input impedance is lowas compared the transmission line.

\ This may be accomplished by employing as the input transformer a step down transformer whose input impedance, of course,

does not match the impedance of the line.

Similar results can, of course, be obtained by employing a step up input transformer to the line impedance, but the step down input transformer is preferred.

Since the repeater gain is substantially independent of changes in line impedance, the repeater may be employed for two way operation and connected to the line without employing the usual hybrid coils and balancing networks. Furthermore in multiplex carrier signaling systems it' is not neccssary to employ the usual directional filters to separate the oppositely directed transmissions at the repeater.

Obviously the repeater may be employed in systems wherein the number of channels are more or less than that shown in Fig. 1.

The repeater of Fig. 2 is similar to that of Fig. 1 but difi'ers therefrom principally inthat capacitative instead of inductive couplings are employed in its inputand output circuits.-

Thi-s repeater comprises an electronic amplifier A of the usual three electrode type having in its input circuit series coupling condensers 10 and 11, the variable resistance network 5, and a shunt resistance 12 and in its output circuit series coupling condensers 13 and 14.

The coupling condensers should be of large capacity so as to have as low &- capacitative reactance as possible.

The repeater may be associated with the transmission line ML by connecting the input circuit conductors 15 and 16 and the output circuit conductors 17 and 18 directly thereto with, conductors 15 and 18 connected to one line conductor and conductors 16 and 17 to the other line conductor. When so connected, the amplifier of the repeater is" included in a loop circuit which includes as a portion thereof a very short length of 'the line conductors. The repeater of Fig. 2

.when connected to the transmission line in this manner functions, in the manner exglained in connection with the repeater of.

ig. 1, to amplify energy incoming thereto from either direction over the transmission me. r

' .Theinvention set forth herein is of course susceptible of various and adaptations.

The invention claimed is:

1. In a signaling system including a transmission line, the method of employing a one way repeater for two way operation which comprises impressing 'part of the amplified ener upon the transmission line, 1mpressing t e energy received from the transmission line and another part of the amplified energy on the input of the repeater to control the amplification of the received energy, and attenuating the energy impressed on the input sufliciently to prevent other modifications line over which signaling currents are transmitted in opposite directions, and a repeater having input and output circuits included in a loop circuit associated with said transmission line, said repeater having. an input impedance which differs by a considerable amount from the impedance of the line.

4. In a signaling system, a transmission line over which signaling currents are transmitted 1n opposite directions, and a repeater having input and output circuits included in a loop circuit associated with said trans mission line,- said re eater having an input impedance greater than the impedance of the line.

5. In a signaling system, a. transmission .line extending between geographically separated stations and over which signals are to be transmitted in opposite directions, and means located at an intermediate pointalong the transmission line for amplifying signals transmitted in'either direction, said means comprising an electronic amplifier having an input circuit and an output circuit, an energy feed back coupling from the output to the input circuits whereby the fed back energy is insulficient to cause the amplifier to generate oscillations, said input circuit being coupled to the transmission line and having therein an input transformer whose input impedance difi'ers very materiallyfrom the impedance of the line. '6. In a signaling system, a transmission line extending between geo raphically separated stations and over w ich signals are to be transmitted in opposite directions, and means located at an intermediate oint along the transmission line for ampli ying signals transmitted in either direction, said means comprising an electronic amplifier .having an input circuit and an output cirwhlch signals are to be transmitted in opposite directions and means located at an intermediate point along the transmission line for amplifying the transmitted signals, said means comprising an electronic valve having an input circuit and an output clrcult, coupling means for feedin energy from the output circuit back to t e input circuit less than the amount necessary to generate susquency line, a plurality of detectors contained oscillations, and means for connecting the input and output circuits in shunt of the transmission line.

8. In a carrier current system, a high frenected thereto, a balancing network individual to each detector. for balancing the high frequency line at a given range of frequencies corresponding in width to that of only one side band of frequencies, an individual band filter "connecting each detector and the associated balancing network to the h gh frequency line, a modulator associated with each detector, anda low pass filter associated with each modulator and detector to prevent singing at frequencies not balanced I by the network. v

9. Ina multiplex telephone system, a high frequency line for-carrying high frequency currents of different frequencies each modulated in accordance with speech; a plurality of band filters, of respectively difi'erent frequency transmission ranges each range corresponding in width to that of essential speech frequencies, directly connected 'to the line; an individual net circuit associated with each band filter for balancing the line for a range of frequencies corresponding in width to only one side band of frequencies transmitted band filter; telephone ni tecting means connected between each filter and its associated net circuit; and an individual low pass filter associated with each b its associated odiilating and demodulating and detecting means toprevent v singing at frequencies not balanced by the network. v

'10. In a multiplex telephones stem, a high frequency line for carrying igh' frequency currents of different frequencies each 1 modulated in accordance with speech; a plu rality of band filters, of respectively different frequency transmission ranges each range corresponding in width to that of es ntial speech fre uencies, connected to the H52; an lndividua net circuit associated with each band filter for balancing the line for the range of frequencies transmitted by its associated band filter; telephone modulating and detecting means connected between each filter and its associated net circuit; and

an individual low pass filter associated with the modulating and detecting means to prevent singing at frequencies not balanced by the net circuits.

In witness whereof, I hereunto subscribe my name this 11 da of July A. D., 1923.

J COB, s JAMMER. 

